Channel quality measurement method in multiple antenna wireless communication system and device for same

ABSTRACT

The present invention relates to a method and device for measuring channel quality by a base station having a two-dimensional active antenna system including multiple antennas. Particularly, the method comprises: receiving channel state information (CSI) generated on the basis of a first reference signal relating to a part of multiple antennas, from a terminal; selecting a precoding and a rank based on a precoding matrix indicator (PMI) and a rank indicator (RI) of the received CSI; generating a port by applying the selected precoding and the selected rank; through the generated port, transmitting, to the terminal, a physical downlink shared channel (PDSCH) and a demodulation-reference signal (DM-RS) configured for the terminal; and receiving, from the terminal, a channel quality indication (CQI) feedback for reducing a mismatch for the CQI of the CSI, wherein the CQI feedback may be generated based on the DM-RS.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly, to a method of transmitting a signal in amulti-antenna wireless communication system and apparatus for the same.

BACKGROUND ART

Multiple input multiple output (MIMO) increases the efficiency of datatransmission and reception using multiple transmit antennas and multiplereceive antennas instead of a single transmission antenna and a singlereception antenna. A receiver receives data through multiple paths whenmultiple antennas are used, whereas the receiver receives data through asingle antenna path when a single antenna is used. Accordingly, MIMO canincrease a data transmission rate and throughput and improve coverage.

A single cell MIMO scheme can be classified into a single user-MIMO(SU-MIMO) scheme for receiving a downlink signal by a single UE in onecell and a multi user-MIMO (MU-MIMO) scheme for receiving a downlinksignal by two or more UEs.

Channel estimation refers to a procedure for compensating for signaldistortion due to fading to restore a reception signal. Here, the fadingrefers to sudden fluctuation in signal intensity due to multipath-timedelay in a wireless communication system environment. For channelestimation, a reference signal (RS) known to both a transmitter and areceiver is required. In addition, the RS can be referred to as a RS ora pilot signal according to applied standard.

A downlink RS is a pilot signal for coherent demodulation for a physicaldownlink shared channel (PDSCH), a physical control format indicatorchannel (PCFICH), a physical hybrid indicator channel (PHICH), aphysical downlink control channel (PDCCH), etc. A downlink RS includes acommon RS (CRS) shared by all user equipments (UEs) in a cell and adedicated RS (DRS) for a specific UE. For a system (e.g., a systemhaving extended antenna configuration LTE-A standard for supporting 8transmission antennas) compared with a conventional communication system(e.g., a system according to LTE release-8 or 9) for supporting 4transmission antennas, DRS based data demodulation has been consideredfor effectively managing RSs and supporting a developed transmissionscheme. That is, for supporting data transmission through extendedantennas, DRS for two or more layers can be defined. DRS is pre-coded bythe same pre-coder as a pre-coder for data and thus a receiver caneasily estimate channel information for data demodulation withoutseparate precoding information.

A downlink receiver can acquire pre-coded channel information forextended antenna configuration through DRS but requires a separate RSother than DRS in order to non-pre-coded channel information.Accordingly, a receiver of a system according to LTE-A standard candefine a RS for acquisition of channel state information (CSI), that is,CSI-RS.

DISCLOSURE OF THE INVENTION Technical Task

The technical task of the present invention is to provide a method ofmeasuring a channel quality in a wireless communication system andapparatus for the same.

It will be appreciated by persons skilled in the art that the objectsthat could be achieved with the present invention are not limited towhat has been particularly described hereinabove and the above and otherobjects that the present invention could achieve will be more clearlyunderstood from the following detailed description.

Technical Solutions

In a first technical aspect of the present invention, provided herein isa method of measuring a channel quality, which is measured by a basestation equipped with a two-dimensional active antenna system includinga plurality of antennas, including: receiving, from a user equipment,CSI (channel state information) on the plurality of the antennas, whichis generated based on a first reference signal for some of the pluralityof the antennas; selecting a precoding and a rank based on a PMI(precoding matrix indicator) and an RI (rank indicator) of the receivedCSI, respectively; generating a port by applying the selected precodingaccording to the selected rank; transmitting, to the user equipment, aDM-RS (demodulation-reference signal) and a PDSCH (physical downlinkshared channel) configured for the user equipment through the generatedport; and receiving, from the user equipment, CQI (channel qualityindicator) feedback for reducing a CQI mismatch in the CSI. In thiscase, the CQI feedback may be generated based on the DM-RS.

Further, the CQI feedback may include a CQI value calculated based onthe DM-RS.

Further, the CQI feedback may include differences between the CQI valueand an MCS (modulation and coding scheme) level calculated based on theDM-RS and a CQI value and an MCS level calculated based on the firstreference signal.

Additionally, the method may further include transmitting, to the userequipment, information indicating presence of CQI feedback transmissionby using DCI (downlink control information) associated with the PDSCH.

Additionally, the method may further include transmitting, to the userequipment, information indicating presence of CQI feedback transmissionby using DCI (downlink control information) for uplink.

In a second technical aspect of the present invention, provided hereinis a method of measuring a channel quality, which is measured by a basestation equipped with a two-dimensional active antenna system includinga plurality of antennas, including: receiving, from a user equipment,CSI (channel state information) on the plurality of the antennas, whichis generated based on a first reference signal for some of the pluralityof the antennas; selecting a precoding and a rank based on a PMI(precoding matrix indicator) and an RI (rank indicator) of the receivedCSI, respectively; generating a port by applying the selected precodingaccording to the selected rank; transmitting, to the user equipment, aCSI-RS (CSI-reference signal) through the generated port; and receiving,from the user equipment, CQI (channel quality indicator) feedback forreducing a CQI mismatch in the CSI. In this case, the CQI feedback maybe generated based on the CSI-RS.

Advantageous Effects

According to embodiments of the present invention, a method for moreaccurately measuring a channel quality in a multi-antenna system andapparatus for the same can be provided.

It will be appreciated by persons skilled in the art that that theeffects achieved by the present invention are not limited to what hasbeen particularly described hereinabove and other advantages of thepresent invention will be more clearly understood from the followingdetailed description.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention:

FIG. 1 is a diagram showing a network structure of an Evolved UniversalMobile Telecommunications System (E-UMTS) as an example of a wirelesscommunication system;

FIG. 2 is a block diagram illustrating configurations of a base station205 and a user equipment 210 in a wireless communication system 200according to the present invention;

FIG. 3 is a diagram for a configuration of a general MIMO communicationsystem;

FIG. 4 is a diagram for an example of a general CDD structure in a MIMOsystem;

FIG. 5 is a diagram illustrating legacy CRS and DRS patterns;

FIG. 6 is a diagram illustrating an example of a DM RS pattern;

FIG. 7 is a diagram illustrating examples of a CSI-RS pattern;

FIG. 8 is a diagram for explaining an example of a scheme ofperiodically transmitting a CSI-RS;

FIG. 9 is a diagram for explaining an example of a scheme ofaperiodically transmitting a CSI-RS;

FIG. 10 is a diagram for explaining an example of using two CSI-RSconfigurations;

FIG. 11 is a diagram for an active antenna system (AAS); and

FIG. 12 illustrates a CQI-mismatch in the related art.

BEST MODE FOR INVENTION

Hereinafter, the preferred embodiments of the present invention will bedescribed with reference to the accompanying drawings. It is to beunderstood that the detailed description, which will be disclosed alongwith the accompanying drawings, is intended to describe the exemplaryembodiments of the present invention, and is not intended to describe aunique embodiment with which the present invention can be carried out.The following detailed description includes detailed matters to providefull understanding of the present invention. However, it will beapparent to those skilled in the art that the present invention can becarried out without the detailed matters. For example, the followingdetailed description is given under the assumption that 3GPP LTE mobilecommunication systems are used. However, the description may be appliedto any other mobile communication system except for specific featuresinherent to the 3GPP LTE systems.

In some cases, to prevent the concept of the present invention frombeing ambiguous, structures and apparatuses of the known art will beomitted, or will be shown in the form of a block diagram based on mainfunctions of each structure and apparatus. Also, wherever possible, thesame reference numbers will be used throughout the drawings and thespecification to refer to the same or like parts.

Moreover, in the following description, it is assumed that a terminalrefers to a mobile or fixed type user equipment such as a user equipment(UE), and an advanced mobile station (AMS). Also, it is assumed that abase station refers to a random node of a network terminal, such as NodeB, eNode B, and an access point (AP), which performs communication withthe user equipment.

In a mobile communication system, a user equipment may receiveinformation from a base station through a downlink and transmitinformation to the base station through an uplink. The information thatthe user equipment transmits or receives includes data and various typesof control information. There are various physical channels according tothe types and usages of information that the user equipment transmits orreceives.

As an example of a mobile communication system to which the presentinvention is applicable, a 3rd Generation Partnership Project Long TermEvolution (hereinafter, referred to as LTE) communication system isdescribed in brief.

FIG. 1 is a view schematically illustrating a network structure of anE-UMTS as an exemplary radio communication system.

An Evolved Universal Mobile Telecommunications System (E-UMTS) is anadvanced version of a conventional Universal Mobile TelecommunicationsSystem (UMTS) and basic standardization thereof is currently underway inthe 3GPP. E-UMTS may be generally referred to as a Long Term Evolution(LTE) system. For details of the technical specifications of the UMTSand E-UMTS, reference can be made to Release 7 and Release 8 of “3rdGeneration Partnership Project; Technical Specification Group RadioAccess Network”.

Referring to FIG. 1, the E-UMTS includes a User Equipment (UE), eNode Bs(eNBs), and an Access Gateway (AG) which is located at an end of thenetwork (E-UTRAN) and connected to an external network. The eNBs maysimultaneously transmit multiple data streams for a broadcast service, amulticast service, and/or a unicast service.

One or more cells may exist per eNB. The cell is set to operate in oneof bandwidths such as 1.25, 2.5, 5, 10, 15, and 20 MHz and provides adownlink (DL) or uplink (UL) transmission service to a plurality of UEsin the bandwidth. Different cells may be set to provide differentbandwidths. The eNB controls data transmission or reception to and froma plurality of UEs. The eNB transmits DL scheduling information of DLdata to a corresponding UE so as to inform the UE of a time/frequencydomain in which the DL data is supposed to be transmitted, coding, adata size, and hybrid automatic repeat and request (HARQ)-relatedinformation.

In addition, the eNB transmits UL scheduling information of UL data to acorresponding UE so as to inform the UE of a time/frequency domain whichmay be used by the UE, coding, a data size, and HARQ-relatedinformation. An interface for transmitting user traffic or controltraffic may be used between eNBs. A core network (CN) may include the AGand a network node or the like for user registration of UEs. The AGmanages the mobility of a UE on a tracking area (TA) basis. One TAincludes a plurality of cells.

Although wireless communication technology has been developed to LTEbased on wideband code division multiple access (WCDMA), the demands andexpectations of users and service providers are on the rise. Inaddition, considering other radio access technologies under development,new technological evolution is required to secure high competitivenessin the future. Decrease in cost per bit, increase in serviceavailability, flexible use of frequency bands, a simplified structure,an open interface, appropriate power consumption of UEs, and the likeare required.

Recently, 3GPP has standardized technology subsequent to LTE. In thisspecification, the technology will be referred to as “LTE-Advanced” or“LTE-A”. A main difference between the LTE system and the LTE-A systemis a system bandwidth. The LTE-A system aims to support a wideband of upto 100 MHz. To achieve this, the LTE-A system employs carrieraggregation or bandwidth aggregation that accomplishes a wideband usinga plurality of frequency blocks. Carrier aggregation uses a plurality offrequency blocks as a large logical frequency band in order to achieve awider frequency band. The bandwidth of each frequency block can bedefined on the basis of a system block bandwidth used in the LTE system.Each frequency block is transmitted using a component carrier.

FIG. 2 is a block diagram illustrating configurations of a base station205 and a user equipment 210 in a wireless communication system 200.

Although one base station 205 and one user equipment 210 are shown forsimplification of a wireless communication system 200, the wirelesscommunication system 200 may include one or more base stations and/orone or more user equipments.

Referring to FIG. 2, the base station 105 may include a transmitting(Tx) data processor 215, a symbol modulator 220, a transmitter 225, atransmitting and receiving antenna 230, a processor 280, a memory 285, areceiver 290, a symbol demodulator 295, and a receiving (Rx) dataprocessor 297. The user equipment 210 may include a Tx data processor265, a symbol modulator 270, a transmitter 275, a transmitting andreceiving antenna 235, a processor 255, a memory 260, a receiver 240, asymbol demodulator 255, and an Rx data processor 250. Although theantennas 230 and 235 are respectively shown in the base station 205 andthe user equipment 210, each of the base station 205 and the userequipment 210 includes a plurality of antennas. Accordingly, the basestation 205 and the user equipment 210 according to the presentinvention support a multiple input multiple output (MIMO) system. Also,the base station 205 according to the present invention may support botha single user-MIMO (SU-MIMO) system and a multi user-MIMO (MU-MIMO)system.

On a downlink, the Tx data processor 215 receives traffic data, formatsand codes the received traffic data, interleaves and modulates (orsymbol maps) the coded traffic data, and provides the modulated symbols(“data symbols”). The symbol modulator 220 receives and processes thedata symbols and pilot symbols and provides streams of the symbols.

The symbol modulator 220 multiplexes the data and pilot symbols andtransmits the multiplexed data and pilot symbols to the transmitter 225.At this time, the respective transmitted symbols may be a signal valueof null, the data symbols and the pilot symbols. In each symbol period,the pilot symbols may be transmitted continuously. The pilot symbols maybe frequency division multiplexing (FDM) symbols, orthogonal frequencydivision multiplexing (OFDM) symbols, time division multiplexing (TDM)symbols, or code division multiplexing (CDM) symbols.

The transmitter 225 receives the streams of the symbols and converts thereceived streams into one or more analog symbols. Also, the transmitter225 generates downlink signals suitable for transmission through a radiochannel by additionally controlling (for example, amplifying, filteringand frequency upconverting) the analog signals. Subsequently, thedownlink signals are transmitted to the user equipment through theantenna 230.

In the user equipment 210, the antenna 235 receives the downlink signalsfrom the base station 205 and provides the received signals to thereceiver 240. The receiver 240 controls (for example, filters, amplifiesand frequency downcoverts) the received signals and digitalizes thecontrolled signals to acquire samples. The symbol demodulator 245demodulates the received pilot symbols and provides the demodulatedpilot symbols to the processor 255 to perform channel estimation.

Also, the symbol demodulator 245 receives a frequency responseestimation value for the downlink from the processor 255, acquires datasymbol estimation values (estimation values of the transmitted datasymbols) by performing data demodulation for the received data symbols,and provides the data symbol estimation values to the Rx data processor250. The Rx data processor 250 demodulates (i.e., symbol de-mapping),deinterleaves, and decodes the data symbol estimation values to recoverthe transmitted traffic data.

Processing based on the symbol demodulator 245 and the Rx data processor250 is complementary to processing based on the symbol demodulator 220and the Tx data processor 215 at the base station 205.

On an uplink, the Tx data processor 265 of the user equipment 210processes traffic data and provides data symbols. The symbol modulator270 receives the data symbols, multiplexes the received data symbolswith the pilot symbols, performs modulation for the multiplexed symbols,and provides the streams of the symbols to the transmitter 275. Thetransmitter 275 receives and processes the streams of the symbols andgenerates uplink signals. The uplink signals are transmitted to the basestation 205 through the antenna 235.

The uplink signals are received in the base station 205 from the userequipment 210 through the antenna 230, and the receiver 290 processesthe received uplink signals to acquire samples. Subsequently, the symboldemodulator 295 processes the samples and provides data symbolestimation values and the pilot symbols received for the uplink. The Rxdata processor 297 recovers the traffic data transmitted from the userequipment 210 by processing the data symbol estimation values.

The processors 255 and 280 of the user equipment 210 and the basestation 205 respectively command (for example, control, adjust, manage,etc.) the operation at the user equipment 210 and the base station 205.The processors 255 and 280 may respectively be connected with thememories 260 and 285 that store program codes and data. The memories 260and 285 respectively connected to the processor 280 store operatingsystem, application, and general files therein.

Each of the processors 255 and 280 may be referred to as a controller, amicrocontroller, a microprocessor, and a microcomputer. Meanwhile, theprocessors 255 and 280 may be implemented by hardware, firmware,software, or their combination. If the embodiment of the presentinvention is implemented by hardware, application specific integratedcircuits (ASICs), digital signal processors (DSPs), digital signalprocessing devices (DSPDs), programmable logic devices (PLDs), and fieldprogrammable gate arrays (FPGAs) configured to perform the embodiment ofthe present invention may be provided in the processors 255 and 280.Meanwhile, if the embodiment according to the present invention isimplemented by firmware or software, firmware or software may beconfigured to include a module, a procedure, or a function, whichperforms functions or operations of the present invention. Firmware orsoftware configured to perform the present invention may be provided inthe processors 255 and 280, or may be stored in the memories 260 and 285and driven by the processors 255 and 280.

Layers of a radio interface protocol between the user equipment 110 orthe base station 105 and a wireless communication system (network) maybe classified into a first layer L1, a second layer L2 and a third layerL3 on the basis of three lower layers of OSI (open systeminterconnection) standard model widely known in communication systems. Aphysical layer belongs to the first layer L1 and provides an informationtransfer service using a physical channel. A radio resource control(RRC) layer belongs to the third layer and provides control radioresources between the user equipment and the network. The user equipmentand the base station may exchange RRC messages with each another throughthe RRC layer.

The term, base station used in the present invention may refer to a“cell or sector” when used as a regional concept. A serving base station(or serving cell) may be regarded as a base station which provides mainservices to UEs and may transmit and receive control information on acoordinated multiple transmission point. In this sense, the serving basestation (or serving cell) may be referred to as an anchor base station(or anchor cell). Likewise, a neighboring base station may be referredto as a neighbor cell used as a local concept.

Multiple Antenna System

In the multiple antenna technology, reception of one whole message doesnot depend on a single antenna path. Instead, in the multiple antennatechnology, data fragments received through multiple antennas arecollected and combined to complete data. If the multiple antennatechnology is used, a data transfer rate within a cell region of aspecific size may be improved, or system coverage may be improved whileensuring a specific data transfer rate. In addition, this technology canbe broadly used by mobile communication devices and relays. Due to themultiple antenna technology, restriction on mobile communication trafficbased on a legacy technology using a single antenna can be solved.

FIG. 3(a) shows the configuration of a wireless communication systemincluding multiple antennas. As shown in FIG. 3(a), the number oftransmit (Tx) antennas and the number of Rx antennas respectively toN_(T) and N_(R), a theoretical channel transmission capacity of the MIMOcommunication system increases in proportion to the number of antennas,differently from the above-mentioned case in which only a transmitter orreceiver uses several antennas, so that transmission rate and frequencyefficiency can be greatly increased. In this case, the transfer rateacquired by the increasing channel transmission capacity cantheoretically increase by a predetermined amount that corresponds tomultiplication of a maximum transfer rate (Ro) acquired when one antennais used and a rate of increase (Ri). The rate of increase (Ri) can berepresented by the following equation 1.

R _(i)=min(N _(T) ,N _(R))  [Equation 1]

For example, provided that a MIMO system uses four Tx antennas and fourRx antennas, the MIMO system can theoretically acquire a high transferrate which is four times higher than that of a single antenna system.After the above-mentioned theoretical capacity increase of the MIMOsystem was demonstrated in the mid-1990s, many developers began toconduct intensive research into a variety of technologies which cansubstantially increase data transfer rate using the theoretical capacityincrease. Some of the above technologies have been reflected in avariety of wireless communication standards, for example,third-generation mobile communication or next-generation wireless LAN,etc.

A variety of MIMO-associated technologies have been intensivelyresearched by many companies or developers, for example, research intoinformation theory associated with MIMO communication capacity undervarious channel environments or multiple access environments, researchinto a radio frequency (RF) channel measurement and modeling of the MIMOsystem, and research into a space-time signal processing technology.

Mathematical modeling of a communication method for use in theabove-mentioned MIMO system will hereinafter be described in detail. Ascan be seen from FIG. 7, it is assumed that there are N_(T) Tx antennasand N_(R) Rx antennas. In the case of a transmission signal, a maximumnumber of transmission information pieces is N_(T) under the conditionthat N_(T) Tx antennas are used, so that the transmission informationcan be represented by a specific vector shown in the following equation2.

s=[s ₁ ,s ₂ , . . . ,s _(N) _(T) ]^(T)  [Equation 2]

In the meantime, individual transmission information pieces s₁, s₂, . .. , s_(NT) may have different transmission powers. In this case, if theindividual transmission powers are denoted by P₁, P₂, . . . , P_(NT),transmission information having an adjusted transmission power can berepresented by a specific vector shown in the following equation 3.

ŝ└ŝ ₁ ,ŝ ₂ , . . . ,ŝ _(N) _(T) ┘^(T) =[Ps ₁ ,Ps ₂ , . . . ,Ps _(N) _(T)]^(T)  [Equation 3]

In Equation 3, ŝ is a transmission vector, and can be represented by thefollowing equation 4 using a diagonal matrix P of a transmission power.

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & \; & \; & 0 \\\; & P_{2} & \; & \; \\\; & \; & \ddots & \; \\0 & \; & \; & P_{N_{T}}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\\vdots \\s_{N_{T}}\end{bmatrix}} = {Ps}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

In the meantime, the information vector Ŝ having an adjustedtransmission power is applied to a weight matrix W, so that N_(T)transmission signals x₁, x₂, . . . , x_(NT) to be actually transmittedare configured. In this case, the weight matrix W is adapted to properlydistribute transmission information to individual antennas according totransmission channel situations. The above-mentioned transmissionsignals x₁, x₂, . . . , x_(NT) can be represented by the followingequation 5 using the vector X. Here, W_(ij) denotes a weightcorresponding to i-th Tx antenna and j-th information. W represents aweight matrix or precoding matrix.

$\begin{matrix}{x = {\left\lbrack \begin{matrix}x_{1} \\x_{2} \\\vdots \\x_{i} \\\vdots \\x_{N_{T}}\end{matrix} \right\rbrack = {{\begin{bmatrix}w_{11} & w_{12} & \ldots & w_{1N_{T}} \\w_{12} & w_{12} & \ldots & w_{2N_{T}} \\\vdots & \; & \ddots & \; \\w_{i\; 2} & w_{i\; 2} & \ldots & w_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\w_{N_{T}1} & w_{N_{T}2} & \ldots & w_{N_{T}N_{T}}\end{bmatrix}\begin{bmatrix}{\hat{s}}_{1} \\{\hat{s}}_{2} \\\vdots \\{\hat{s}}_{j} \\\vdots \\{\hat{s}}_{N_{T}}\end{bmatrix}} = {{W\hat{s}} = {WPs}}}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

Given N_(R) Rx antennas, signals received at the respective Rx antennas,y₁, y₂, . . . y_(N) _(x) may be represented as the following vector.

y=[y ₁ ,y ₂ , . . . ,y _(N) _(R) ]^(T)  [Equation 6]

When channels are modeled in the MIMO communication system, they may bedistinguished according to the indexes of Tx and Rx antennas and thechannel between a j^(th) Tx antenna and an i^(th) Rx antenna may berepresented as h_(ij). It is to be noted herein that the index of the Rxantenna precedes that of the Tx antenna in h_(ij).

The channels may be represented as vectors and matrices by groupingthem. Examples of vector expressions are given as below. FIG. 3(b)illustrates channels from N_(T) Tx antennas to an i^(th) Rx antenna.

As illustrated in FIG. 3(b), the channels from the N_(T) Tx antennas toan i^(th) Rx antenna may be expressed as follows.

h _(i) ^(T) =[h _(i1) ,h _(i2) , . . . ,h _(iN) _(T) ][Equation 7]

Also, all channels from the N_(T) Tx antennas to the N_(R) Rx antennasmay be expressed as the following matrix.

$\begin{matrix}{H = {\begin{bmatrix}h_{1}^{T} \\h_{2}^{T} \\\vdots \\h_{i}^{T} \\\vdots \\h_{N_{R}}^{T}\end{bmatrix} = \begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1N_{T}} \\h_{12} & h_{12} & \ldots & h_{2N_{T}} \\\vdots & \; & \ddots & \; \\h_{i\; 2} & h_{i\; 2} & \ldots & h_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}}} & \left\lbrack {{Equation}\mspace{14mu} 8} \right\rbrack\end{matrix}$

Actual channels experience the above channel matrix H and then are addedwith Additive White Gaussian Noise (AWGN). The AWGN n₁, n₂, . . . ,n_(N) _(R) added to the N_(R) Rx antennas is given as the followingvector.

n=[n ₁ ,n ₂ , . . . ,n _(N) _(R) ]^(T)  [Equation 9]

From the above modeled equations, the received signal can be expressedas follows.

$\begin{matrix}{y = {\begin{bmatrix}y_{1} \\y_{2} \\\vdots \\y_{i} \\\vdots \\y_{N_{R}}\end{bmatrix} = {{{\begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1N_{T}} \\h_{12} & h_{12} & \ldots & h_{2N_{T}} \\\vdots & \; & \ddots & \; \\h_{i\; 2} & h_{i\; 2} & \ldots & h_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{j} \\\vdots \\x_{N_{T}}\end{bmatrix}} + \begin{bmatrix}n_{1} \\n_{2} \\\vdots \\n_{i} \\\vdots \\n_{N_{R}}\end{bmatrix}} = {{Hx} + n}}}} & \left\lbrack {{Equation}\mspace{14mu} 10} \right\rbrack\end{matrix}$

In the meantime, the numbers of rows and columns in the channel matrix Hrepresenting channel states are determined according to the numbers ofTx and Rx antennas. The number of rows is identical to that of Rxantennas, N_(R) and the number of columns is identical to that of Txantennas, N_(T). Thus, the channel matrix H is of size N_(R)×N_(T). Ingeneral, the rank of a matrix is defined as the smaller between thenumbers of independent rows and columns. Accordingly, the rank of thematrix is not larger than the number of rows or columns. The rank of thematrix H, rank(H) is limited as follows.

rank(H)≦min(N _(T) ,N _(R))  [Equation 11]

As a multi-antenna transmission and reception scheme used for operatinga multi-antenna system, it may be able to use FSTD (frequency switchedtransmit diversity), SFBC (Space Frequency Block Code), STBC (Space TimeBlock Code), CDD (Cyclic Delay Diversity), TSTD (time switched transmitdiversity) and the like. In a rank 2 or higher, SM (SpatialMultiplexing), GCDD (Generalized Cyclic Delay Diversity), S-VAP(Selective Virtual Antenna Permutation) and the like can be used.

The FSTD corresponds to a scheme of obtaining a diversity gain byassigning a subcarrier of a different frequency to a signal transmittedby each of multiple antennas. The SFBC corresponds to a scheme capableof securing both a diversity gain in a corresponding dimension and amulti-user scheduling gain by efficiently applying selectivity in aspatial domain and a frequency domain. The STBC corresponds to a schemeof applying selectivity in a spatial domain and a time domain. The CDDcorresponds to a scheme of obtaining a diversity gain using path delaybetween transmission antennas. The TSTD corresponds to a scheme ofdistinguishing signals transmitted by multiple antennas from each otheron the basis of time. The spatial multiplexing (SM) corresponds to ascheme of increasing a transfer rate by transmitting a different dataaccording to an antenna. The GCDD corresponds to a scheme of applyingselectivity in a time domain and a frequency domain. The S-VAPcorresponds to a scheme of using a single precoding matrix. The S-VAPcan be classified into an MCW (multi codeword) S-VAP for mixing multiplecodewords between antennas in spatial diversity or spatial multiplexingand an SCW (single codeword) S-VAP for using a single codeword.

Among the aforementioned MIMO transmission schemes, the STBC schemecorresponds to a scheme of obtaining time diversity in a manner that anidentical data symbol is repeated in a time domain to supportorthogonality. Similarly, the SFBC scheme corresponds to a scheme ofobtaining frequency diversity in a manner that an identical data symbolis repeated in a frequency domain to support orthogonality. Examples ofa time block code used for the STBC and a frequency block code used forthe SFBC can be represented as equation 12 and equation 13,respectively. The equation 12 indicates a block code in case of 2transmission antennas and the equation 13 indicates a block code in caseof 4 transmission antennas.

$\begin{matrix}{\frac{1}{\sqrt{2}}\begin{pmatrix}S_{1} & S_{2} \\{- S_{2}^{*}} & S_{1}^{*}\end{pmatrix}} & \left\lbrack {{Equation}\mspace{14mu} 12} \right\rbrack \\{\frac{1}{\sqrt{2}}\begin{pmatrix}S_{1} & S_{2} & 0 & 0 \\0 & 0 & S_{3} & S_{4} \\{- S_{2}^{*}} & S_{1}^{*} & 0 & 0 \\0 & 0 & {- S_{4}^{*}} & S_{3}^{*}\end{pmatrix}} & \left\lbrack {{Equation}\mspace{14mu} 13} \right\rbrack\end{matrix}$

In the equations 12 and 13, Si (i=1, 2, 3, 4) corresponds to a modulateddata symbol. And, in the equations 12 and 13, a row of a matrixcorresponds to an antenna port and a column of the matrix corresponds totime (STBC) or frequency (SFBC).

Meanwhile, among the aforementioned MIMO transmission schemes, the CDDscheme corresponds to a scheme of increasing frequency diversity byincreasing delay propagation on purpose. FIG. 4 shows an example of ageneral CDD structure in a multi-antenna system. FIG. 4 (a) shows ascheme of applying cyclic delay in time domain. As shown in FIG. 4 (b),the CDD scheme applying the cyclic delay of FIG. 4 (a) can also beimplemented by applying phase-shift diversity.

Reference Signals (RSs)

In a wireless communication system, a packet is transmitted on a radiochannel. In view of the nature of the radio channel, the packet may bedistorted during the transmission. To receive the signal successfully, areceiver should compensate for the distortion of the reception signalusing channel information. Generally, to enable the receiver to acquirethe channel information, a transmitter transmits a signal known to boththe transmitter and the receiver and the receiver acquires knowledge ofchannel information based on the distortion of the signal received onthe radio channel. This signal is called a pilot signal or an RS.

In the case of data transmission and reception through multipleantennas, knowledge of channel states between transmission (Tx) antennasand reception (Rx) antennas is required for successful signal reception.Accordingly, an RS should be transmitted through each Tx antenna.

RSs in a mobile communication system may be divided into two typesaccording to their purposes: RS for channel information acquisition andRS for data demodulation. Since its purpose lies in that a UE acquiresdownlink channel information, the former should be transmitted in abroad band and received and measured even by a UE that does not receivedownlink data in a specific subframe. This RS is also used in asituation like handover. The latter is an RS that an eNB transmits alongwith downlink data in specific resources. A UE can estimate a channel byreceiving the RS and accordingly can demodulate data. The RS should betransmitted in a data transmission area.

A legacy 3GPP LTE (e.g., 3GPP LTE release-8) system defines two types ofdownlink RSs for unicast services: a common RS (CRS) and a dedicated RS(DRS). The CRS is used for acquisition of information about a channelstate, measurement of handover, etc. and may be referred to as acell-specific RS. The DRS is used for data demodulation and may bereferred to as a UE-specific RS. In a legacy 3GPP LTE system, the DRS isused for data demodulation only and the CRS can be used for bothpurposes of channel information acquisition and data demodulation.

CRSs, which are cell-specific, are transmitted across a wideband inevery subframe. According to the number of Tx antennas at an eNB, theeNB may transmit CRSs for up to four antenna ports. For instance, an eNBwith two Tx antennas transmits CRSs for antenna port 0 and antennaport 1. If the eNB has four Tx antennas, it transmits CRSs forrespective four Tx antenna ports, antenna port 0 to antenna port 3.

FIG. 5 illustrates a CRS and DRS pattern for an RB (including 14 OFDMsymbols in time by 12 subcarriers in frequency in case of a normal CP)in a system where an eNB has four Tx antennas. In FIG. 5, REs labeledwith ‘R0’, ‘R1’, ‘R2’ and ‘R3’ represent the positions of CRSs forantenna port 0 to antenna port 4, respectively. REs labeled with ‘D’represent the positions of DRSs defined in the LTE system.

The LTE-A system, an evolution of the LTE system, can support up toeight Tx antennas. Therefore, it should also support RSs for up to eightTx antennas. Because downlink RSs are defined only for up to four Txantennas in the LTE system, RSs should be additionally defined for fiveto eight Tx antenna ports, when an eNB has five to eight downlink Txantennas in the LTE-A system. Both RSs for channel measurement and RSsfor data demodulation should be considered for up to eight Tx antennaports.

One of significant considerations for design of the LTE-A system isbackward compatibility. Backward compatibility is a feature thatguarantees a legacy LTE terminal to operate normally even in the LTE-Asystem. If RSs for up to eight Tx antenna ports are added to atime-frequency area in which CRSs defined by the LTE standard aretransmitted across a total frequency band in every subframe, RS overheadbecomes huge. Therefore, new RSs should be designed for up to eightantenna ports in such a manner that RS overhead is reduced.

Largely, new two types of RSs are introduced to the LTE-A system. Onetype is CSI-RS serving the purpose of channel measurement for selectionof a transmission rank, a modulation and coding scheme (MCS), aprecoding matrix index (PMI), etc. The other type is demodulation RS (DMRS) for demodulation of data transmitted through up to eight Txantennas.

Compared to the CRS used for both purposes of measurement such aschannel measurement and measurement for handover and data demodulationin the legacy LTE system, the CSI-RS is designed mainly for channelestimation, although it may also be used for measurement for handover.Since CSI-RSs are transmitted only for the purpose of acquisition ofchannel information, they may not be transmitted in every subframe,unlike CRSs in the legacy LTE system. Accordingly, CSI-RSs may beconfigured so as to be transmitted intermittently (e.g. periodically)along the time axis, for reduction of CSI-RS overhead.

When data is transmitted in a downlink subframe, DM RSs are alsotransmitted dedicatedly to a UE for which the data transmission isscheduled. Thus, DM RSs dedicated to a particular UE may be designedsuch that they are transmitted only in a resource area scheduled for theparticular UE, that is, only in a time-frequency area carrying data forthe particular UE.

FIG. 6 illustrates an exemplary DM RS pattern defined for the LTE-Asystem. In FIG. 6, the positions of REs carrying DM RSs in an RBcarrying downlink data (an RB having 14 OFDM symbols in time by 12subcarriers in frequency in case of a normal CP) are marked. DM RSs maybe transmitted for additionally defined four antenna ports, antenna port7 to antenna port 10 in the LTE-A system. DM RSs for different antennaports may be identified by their different frequency resources(subcarriers) and/or different time resources (OFDM symbols). This meansthat the DM RSs may be multiplexed in frequency division multiplexing(FDM) and/or time division multiplexing (TDM). If DM RSs for differentantenna ports are positioned in the same time-frequency resources, theymay be identified by their different orthogonal codes. That is, these DMRSs may be multiplexed in Code Division Multiplexing (CDM). In theillustrated case of FIG. 6, DM RSs for antenna port 7 and antenna port 8may be located on REs of DM RS CDM group 1 through multiplexing based onorthogonal codes. Similarly, DM RSs for antenna port 9 and antenna port10 may be located on REs of DM RS CDM group 2 through multiplexing basedon orthogonal codes.

FIG. 7 illustrates exemplary CSI-RS patterns defined for the LTE-Asystem. In FIG. 7, the positions of REs carrying CSI-RSs in an RBcarrying downlink data (an RB having 14 OFDM symbols in time by 12subcarriers in frequency in case of a normal CP) are marked. One of theCSI-RS patterns illustrated in FIGS. 7(a) to 7(e) is available for anydownlink subframe. CSI-RSs may be transmitted for eight antenna portssupported by the LTE-A system, antenna port 15 to antenna port 22.CSI-RSs for different antenna ports may be identified by their differentfrequency resources (subcarriers) and/or different time resources (OFDMsymbols). This means that the CSI-RSs may be multiplexed in FDM and/orTDM. CSI-RSs positioned in the same time-frequency resources fordifferent antenna ports may be identified by their different orthogonalcodes. That is, these DM RSs may be multiplexed in CDM. In theillustrated case of FIG. 7(a), CSI-RSs for antenna port 15 and antennaport 16 may be located on REs of CSI-RS CDM group 1 through multiplexingbased on orthogonal codes. CSI-RSs for antenna port 17 and antenna port18 may be located on REs of CSI-RS CDM group 2 through multiplexingbased on orthogonal codes. CSI-RSs for antenna port 19 and antenna port20 may be located on REs of CSI-RS CDM group 3 through multiplexingbased on orthogonal codes. CSI-RSs for antenna port 21 and antenna port22 may be located on REs of CSI-RS CDM group 4 through multiplexingbased on orthogonal codes. The same principle described with referenceto FIG. 7(a) is applicable to the CSI-RS patterns illustrated in FIGS.7(b) to 7(e).

RS patterns shown in FIGS. 5 to 7 are disclosed only for illustrativepurposes, and the scope or spirit of the present invention are notlimited only to a specific RS pattern. That is, even in the case inwhich RS patterns different from those of FIGS. 5 to 7 are defined andused, various embodiments of the present invention can also be equallyapplied thereto without difficulty.

CSI-RS Configuration

Among a plurality of CSI-RSs and a plurality of IMRs set to a UE, oneCSI process can be defined in a manner of associating a CSI-RS resourcefor measuring a signal with an interference measurement resource (IMR)for measuring interference. A UE feedbacks CSI information induced fromCSI processes different from each other to a network (e.g., basestation) with an independent period and a subframe offset.

In particular, each CSI process has an independent CSI feedbackconfiguration. The base station can inform the UE of the CS-RS resource,the IMR resource association information and the CSI feedbackconfiguration via higher layer signaling. For example, assume that threeCSI processes shown in Table 1 are set to the UE.

TABLE 1 Signal Measurement CSI Process Resource (SMR) IMR CSI process 0CSI-RS 0 IMR 0 CSI process 1 CSI-RS 1 IMR 1 CSI process 2 CSI-RS 0 IMR 2

In Table 1, a CSI-RS 0 and a CSI-RS 1 indicate a CSI-RS received from acell 1 corresponding to a serving cell of a UE and a CSI-RS receivedfrom a cell 2 corresponding to a neighbor cell participating incooperation, respectively. IMRs set to each of the CSI processes shownin Table 1 are shown in Table 2.

TABLE 2 IMR eNB 1 eNB 2 IMR 0 Muting Data transmission IMR 1 Datatransmission Muting IMR 2 Muting Muting

A cell 1 performs muting in an IMR 0 and a cell 2 performs datatransmission in the IMR 0. A UE is configured to measure interferencefrom other cells except the cell 1 in the IMR 0. Similarly, the cell 2performs muting in an IMR 1 and the cell 1 performs data transmission inthe IMR 1. The UE is configured to measure interference from other cellsexcept the cell 2 in the IMR 1. The cell 1 and the cell 2 perform mutingin an IMR 2 and the UE is configured to measure interference from othercells except the cell 1 and the cell 2 in the IMR 2.

Hence, as shown in Table 1 and Table 2, if data is received from thecell 1, CSI information of the CSI process 0 indicates optimized RI, PMIand CQI information. If data is received from the cell 2, CSIinformation of the CSI process 1 indicates optimized RI, PMI and CQIinformation. If data is received from the cell 1 and there is nointerference from the cell 2, CSI information of the CSI process 2indicates optimized RI, PMI and CQI information.

It is preferable for a plurality of CSI processes set to a UE to sharevalues subordinate to each other. For example, in case of jointtransmission performed by the cell 1 and the cell 2, if a CSI process 1considering a channel of the cell 1 as a signal part and a CSI process 2considering a channel of the cell 2 as a signal part are set to a UE, itis able to easily perform JT scheduling only when ranks of the CSIprocess 1 and the CSI process 2 and a selected subband index areidentical to each other.

A period or a pattern of transmitting a CSI-RS can be configured by abase station. In order to measure the CSI-RS, a UE should be aware ofCSI-RS configuration of each CSI-RS antenna port of a cell to which theUE belongs thereto. The CSI-RS configuration can include a DL subframeindex in which the CSI-RS is transmitted, time-frequency location of aCSI-RS resource element (RE) in a transmission subframe (e.g., theCSI-RS patterns shown in FIGS. 7(a) to 7(e)) and a CSI-RS sequence (asequence used for a CSI-RS usage, the sequence is pseudo-randomlygenerated according to a prescribed rule based on a slot number, a cellID, a CP length and the like), etc. In particular, a plurality of CSI-RSconfigurations can be used by a random (given) base station and the basestation can inform a UE(s) in a cell of a CSI-RS configuration to beused for the UE(s).

Since it is necessary to identify a CSI-RS for each antenna port,resources to which the CSI-RS for each antenna port is transmittedshould be orthogonal to each other. As mentioned earlier with referenceto FIG. 7, the CSI-RS for each antenna port can be multiplexed by theFDM, the TDM and/or the CDM scheme using an orthogonal frequencyresource, an orthogonal time resource and/or an orthogonal coderesource.

When the base station informs the UEs in a cell of information on aCSI-RS (CSI-RS configuration), it is necessary for the base station topreferentially inform the UEs of information on time-frequency to whichthe CSI-RS for each antenna port is mapped. Specifically, information ontime can include numbers of subframes in which a CSI-RS is transmitted,a period of transmitting a CSI-RS, a subframe offset of transmitting aCSI-RS, an OFDM symbol number in which a CSI-RS resource element (RE) ofa specific antenna is transmitted, etc. Information on frequency caninclude a frequency space of transmitting a CSI-RS resource element (RE)of a specific antenna, an RE offset on a frequency axis, a shift value,etc.

FIG. 8 is a diagram for explaining an example of a scheme ofperiodically transmitting a CSI-RS. A CSI-RS can be periodicallytransmitted with a period of an integer multiple of a subframe (e.g.,5-subframe period, 10-subframe period, 20-subframe period, 40-subframeperiod or 80-subframe period).

FIG. 8 shows a radio frame configured by 10 subframes (subframe number 0to 9). In FIG. 8, for example, a transmission period of a CSI-RS of abase station corresponds to 10 ms (i.e., 10 subframes) and a CSI-RStransmission offset corresponds to 3. The offset value may varydepending on a base station to make CSI-RSs of many cells to be evenlydistributed in time domain. If a CSI-RS is transmitted with a period of10 ms, an offset value may have one selected from among 0 to 9.Similarly, if a CSI-RS is transmitted with a period of 5 ms, an offsetvalue may have one selected from among 0 to 4. If a CSI-RS istransmitted with a period of 20 ms, an offset value may have oneselected from among 0 to 19. If a CSI-RS is transmitted with a period of40 ms, an offset value may have one selected from among 0 to 39. If aCSI-RS is transmitted with a period of 80 ms, an offset value may haveone selected from among 0 to 79. The offset value corresponds to a valueof a subframe in which CSI-RS transmission starts by a base stationtransmitting a CSI-RS with a prescribed period. If the base stationinforms a UE of a transmission period of a CSI-RS and an offset value,the UE is able to receive the CSI-RS of the base station at acorresponding subframe position using the transmission period and theoffset value. The UE measures a channel through the received CSI-RS andmay be then able to report such information as a CQI, a PMI and/or an RI(rank indicator) to the base station. In the present disclosure, theCQI, the PMI and/or the RI can be commonly referred to as CQI (or CSI)except a case of individually explaining the CQI, the PMI and/or the RI.And, the CSI-RS transmission period and the offset can be separatelydesignated according to a CSI-RS configuration.

FIG. 9 is a diagram for explaining an example of a scheme ofaperiodically transmitting a CSI-RS. In FIG. 9, for example, one radioframe is configured by 10 subframes (subframe number 0 to 9). As shownin FIG. 9, a subframe in which a CSI-RS is transmitted can berepresented as a specific pattern. For example, a CSI-RS transmissionpattern can be configured by a 10-subframe unit and whether to transmita CSI-RS can be indicated by a 1-bit indicator in each subframe. Anexample of FIG. 9 shows a pattern of transmitting a CSI-RS in a subframeindex 3 and 4 among 10 subframes (subframe index 0 to 9). The indicatorcan be provided to a UE via higher layer signaling.

As mentioned in the foregoing description, configuration of CSI-RStransmission can be variously configured. In order to make a UE properlyreceive a CSI-RS and perform channel measurement, it is necessary for abase station to inform the UE of CSI-RS configuration. Embodiments ofthe present invention for informing a UE of CSI-RS configuration areexplained in the following.

Method of Indicating CSI-RS Configuration

In general, a base station is able to inform a UE of CSI-RSconfiguration by one of two schemes in the following.

A first scheme is a scheme that a base station broadcasts information onCSI-RS configuration to UEs using dynamic broadcast channel (DBCH)signaling.

In a legacy LTE system, when a base station informs UEs of contents onsystem information, the information is transmitted to the UEs via a BCH(broadcasting channel). Yet, if the contents are too much and the BCH isunable to carry all of the contents, the base station transmits thesystem information using a scheme used for transmitting a generaldownlink data. And, PDCCH CRC of corresponding data is transmitted in amanner of being masked using SI-RNTI, i.e., system information RNTI,instead of a specific UE ID (e.g., C-RNTI). In this case, actual systeminformation is transmitted to a PDSCH region together with a generalunicast data. By doing so, all UEs in a cell decode PDCCH using theSI-RNTI, decode PDSCH indicated by the corresponding PDCCH and may bethen able to obtain the system information. This type of broadcastingscheme may be referred to as a DBCH (dynamic BCH) to differentiate itfrom a general broadcasting scheme, i.e., PBCH (physical BCH).

Meanwhile, system information broadcasted in a legacy LTE system can bedivided into two types. One is a master information block (MIB)transmitted on the PBCH and another one is a system information block(SIB) transmitted on a PDSCH region in a manner of being multiplexedwith a general unicast data. In the legacy LTE system, sinceinformations transmitted with an SIB type 1 to an SIB type 8 (SIB1 toSIB8) are already defined, it may be able to define a new SIB type totransmit information on a CSI-RS configuration corresponding to newsystem information not defined in the legacy SIB types. For example, itmay be able to define SIB9 or SIB10 and the base station can inform UEswithin a cell of the information on the CSI-RS configuration via theSIBS or the SIB10 using a DBCH scheme.

A second scheme is a scheme that a base station informs each UE ofinformation on CSI-RS configuration using RRC (radio resource control)signaling. In particular, the information on the CSI-RS can be providedto each of the UEs within a cell using dedicated RRC signaling. Forexample, in the course of establishing a connection with the basestation via an initial access or handover of a UE, the base station caninform the UE of the CSI-RS configuration via RRC signaling. Or, whenthe base station transmits an RRC signaling message, which requireschannel status feedback based on CSI-RS measurement, to the UE, the basestation can inform the UE of the CSI-RS configuration via the RRCsignaling message.

Indication of CSI-RS Configuration

A random base station may use a plurality of CSI-RS configurations andthe base station can transmit a CSI-RS according to each of a pluralityof the CSI-RS configurations to a UE in a predetermined subframe. Inthis case, the base station informs the UE of a plurality of the CSI-RSconfigurations and may be able to inform the UE of a CSI-RS to be usedfor measuring a channel state for making a feedback on a CQI (channelquality information) or CSI (channel state information).

Embodiments for a base station to indicate a CSI-RS configuration to beused in a UE and a CSI-RS to be used for measuring a channel areexplained in the following.

FIG. 10 is a diagram for explaining an example of using two CSI-RSconfigurations. In FIG. 10, for example, one radio frame is configuredby 10 subframes (subframe number 0 to 9). In FIG. 10, in case of a firstCSI-RS configuration, i.e., a CSI-RS1, a transmission period of a CSI-RSis 10 ms and a transmission offset of a CSI-RS is 3. In FIG. 10, in caseof a second CSI-RS configuration, i.e., a CSI-RS2, a transmission periodof a CSI-RS is 10 ms and a transmission offset of a CSI-RS is 4. A basestation informs a UE of information on two CSI-RS configurations and maybe able to inform the UE of a CSI-RS configuration to be used for CQI(or CSI) feedback among the two CSI-RS configurations.

If the base station asks the UE to make a CQI feedback on a specificCSI-RS configuration, the UE can perform channel state measurement usinga CSI-RS belonging to the CSI-RS configuration only. Specifically, achannel state is determined based on CSI-RS reception quality, an amountof noise/interference and a function of a correlation coefficient. Inthis case, the CSI-RS reception quality is measured using the CSI-RSbelonging to the CSI-RS configuration only. In order to measure theamount of noise/interference and the correlation coefficient (e.g., aninterference covariance matrix indicating interference direction, etc.),measurement can be performed in a subframe in which the CSI-RS istransmitted or a subframe designated in advance. For example, in theembodiment of FIG. 10, if the base station asks the UE to make afeedback on the first CSI-RS configuration (CSI-RS1), the UE measuresreception quality using a CSI-RS transmitted in a fourth subframe (asubframe index 3) of a radio frame and the UE can be separatelydesignated to use an add number subframe to measure the amount ofnoise/interference and the correlation coefficient. Or, it is able todesignate the UE to measure the CSI-RS reception quality, the amount ofnoise/interference and the correlation coefficient in a specific singlesubframe (e.g., a subframe index 3) only.

For example, reception signal quality measured using a CSI-RS can besimply represented by SINR (signal-to-interference plus noise ratio) asS/(I+N) (in this case, S corresponds to strength of a reception signal,I corresponds to an amount of interference and N corresponds to anamount of noise). The S can be measured through a CSI-RS in a subframeincluding the CSI-RS in a subframe including a signal transmitted to aUE. Since the I and the N change according to an amount of interferencereceived from a neighbor cell, direction of a signal received from aneighbor cell, and the like, the I and the N can be measured by a CRStransmitted in a subframe in which the S is measured or a separatelydesignated subframe, etc.

In this case, the amount of noise/interference and the correlationcoefficient can be measured in a resource element (RE) in which a CRSbelonging to a corresponding subframe or a CSI-RS is transmitted. Or, inorder to easily measure noise/interference, the noise/interference canbe measured through a configured null RE. In order to measurenoise/interference in a CRS or CSI-RS RE, a UE preferentially recovers aCRS or a CSI-RS and subtracts a result of the recovery from a receptionsignal to make a noise and interference signal to be remained only. Bydoing so, the UE is able to obtain statistics of noise/interference fromthe remained noise and the interference signal. A null RE may correspondto an empty RE (i.e., transmission power is 0 (zero)) in which no signalis transmitted by a base station. The null RE makes other base stationsexcept the corresponding base station easily measure a signal. In orderto measure an amount of noise/interference, it may use all of a CRS RE,a CSI-RS RE and a null RE. Or, a base station may designate REs to beused for measuring noise/interference for a UE. This is because it isnecessary to properly designate an RE to be used for measuringnoise/interference measured by the UE according to whether a signal of aneighbor cell transmitted to the RE corresponds to a data signal or acontrol signal. Since the signal of the neighbor cell transmitted to theRE varies according to whether or not synchronization between cells ismatched, a CRS configuration, a CSI-RS configuration and the like, thebase station identifies the signal of the neighbor cell and may be ableto designate an RE in which measurement is to be performed for the UE.In particular, the base station can designate the UE to measurenoise/interference using all or a part of the CRS RE, the CSI-RS RE andthe null RE.

For example, the base station may use a plurality of CSI-RSconfigurations and may be able to inform the UE of a CSI-RSconfiguration to be used for CQI feedback and a null RE position whileinforming the UE of one or more CSI-RS configurations. In order todistinguish the CSI-RS configuration to be used for CQI feedback by theUE from a null RE transmitted by zero transmission power, the CSI-RSconfiguration to be used for CQI feedback by the UE may correspond to aCSI-RS configuration transmitted by non-zero transmission power. Forexample, if the base station informs the UE of a CSI-RS configuration inwhich the UE performs channel measurement, the UE can assume that aCSI-RS is transmitted by non-zero transmission power in the CSI-RSconfiguration. In addition, if the base station informs the UE of aCSI-RS configuration transmitted by zero transmission power (i.e., nullRE position), the UE can assume that an RE position of the CSI-RSconfiguration corresponds to zero transmission power. In other word,when the base station informs the UE of a CSI-RS configuration ofnon-zero transmission power, if there exists a CSI-RS configuration ofzero transmission power, the base station can inform the UE of acorresponding null RE position.

As a modified example of the method of indicating a CSI-RSconfiguration, the base station informs the UE of a plurality of CSI-RSconfigurations and may be able to inform the UE of all or a part ofCSI-RS configurations to be used for CQI feedback among a plurality ofthe CSI-RS configurations. Hence, having received a request for CQIfeedback on a plurality of the CSI-RS configurations, the UE measures aCQI using a CSI-RS corresponding to each CSI-RS configuration and may bethen able to transmit a plurality of CQI information to the basestation.

Or, in order to make the UE transmit a CQI for each of a plurality ofthe CSI-RS configurations, the base station can designate an uplinkresource, which is necessary for the UE to transmit the CQI, in advanceaccording to each CSI-RS configuration. Information on the uplinkresource designation can be provided to the UE in advance via RRCsignaling.

Or, the base station can dynamically trigger the UE to transmit a CQIfor each of a plurality of CSI-RS configurations to the base station.Dynamic triggering of CQI transmission can be performed via PDCCH. Itmay inform the UE of a CSI-RS configuration for which a CQI is to bemeasured via PDCCH. Having received the PDCCH, the UE can feedback a CQImeasurement result measured for the CSI-RS configuration designated bythe PDCCH to the base station.

A transmission timing of a CSI-RS corresponding to each of a pluralityof the CSI-RS configurations can be designated to be transmitted in adifferent subframe or an identical subframe. If CSI-RSs according toCSI-RS configurations different from each other are designated to betransmitted in an identical subframe, it may be necessary to distinguishthe CSI-RSs from each other. In order to distinguish the CSI-RSsaccording to the CSI-RS configurations different from each other, it maybe able to differently apply at least one selected from the groupconsisting of a time resource, a frequency resource and a code resourceof CSI-RS transmission. For example, an RE position in which a CSI-RS istransmitted can be differently designated in a subframe according to aCSI-RS configuration (e.g., a CSI-RS according to one CSI-RSconfiguration is designated to be transmitted in an RE position shown inFIG. 7 (a) and a CSI-RS according to another CSI-RS configuration isdesignated to be transmitted in an RE position shown in FIG. 7 (b))(distinction using a time and frequency resource). Or, if CSI-RSsaccording to CSI-RS configurations different from each other aretransmitted in an identical RE position, the CSI-RSs can bedistinguished from each other by differently using a CSI-RS scramblingcode in the CSI-RS configurations different from each other (distinctionusing a code resource).

Quasi Co-Located (QC)

A UE can receive data from a plurality of transmission points (TPs)(e.g., a TP1 and a TP2). Hence, the UE is able to transmit channel stateinformation on a plurality of the TPs. In this case, RSs can also betransmitted to the UE from a plurality of the TPs. In this case, if itis able to share properties for channel estimation from RS portsdifferent from each other of TPs different from each other, it may beable to reduce load and complexity of reception processing of the UE.Moreover, if it is able to share properties for channel estimation fromRS ports different from each other of an identical TP between the RSports, it may be able to reduce load and complexity of receptionprocessing of the UE. Hence, LTE-A system proposes a method of sharingproperties for channel estimation between RS ports.

For channel estimation between RS ports, LTE-A system has introducedsuch a concept as “quasi co-located (QLC)”. For example, if two antennaports are quasi co-located (QC), the UE may assume that large-scaleproperties of the signal received from the first antenna port can beinferred from the signal received from the other antenna port”. In thiscase, the large-scale properties can include at least one selected fromthe group consisting of delay spread, Doppler spread, Doppler shift,average gain and average delay. In the following, the quasi co-locatedis simply referred to as QCL.

In particular, if two antenna ports are QCL, it may indicate thatlarge-scale properties of a radio channel received from one antenna portare identical to large-scale properties of a radio channel received fromanother antenna port. If antenna ports transmitting RSs different fromeach other are QCL, large-scale properties of a radio channel receivedfrom one antenna port of a type can be replaced with large-scaleproperties of a radio channel received from one antenna port of adifferent type.

According to the aforementioned QCL concept, a UE is unable to assumelarge-scale channel properties identical to each other between radiochannels received from non-QCL (NQC) antenna ports. In particular, inthis case, a UE should perform an independent processing according toeach configured non-QCL antenna port to obtain timing acquisition andtracking, frequency offset estimation and compensation, delayestimation, and Doppler estimation and the like.

A UE can perform operations in the following between antenna portscapable of assuming QCL. First of all, the UE can use delay spread,Doppler spectrum, Doppler spread estimation result for a radio channelreceived from an antenna port when a channel is estimated for a radiochannel received from a different antenna port. Secondly, regardingfrequency shift and received timing, after time synchronization andfrequency synchronization for a single antenna port are performed, theUE can apply identical synchronization to demodulation of a differentantenna port. Thirdly, regarding average received power, the UE canaverage RSRP (reference signal received power) measurements for over twoor more antenna ports.

If a UE receives a DMRS-based DL-related DCI format via a controlchannel (PDCCH or EPDCCH), the UE performs channel estimation for acorresponding PDSCH via a DM-RS sequence and performs data demodulation.If DMRS port configuration received from a DL scheduling grant iscapable of being QCL with a CRS port, the UE can apply the large-scalechannel properties estimation estimated from the CRS port as it is incase of estimating a channel via the DMRS port. This is because a CRScorresponds to a reference signal broadcasted in every subframe withrelatively high density over a whole band, the estimation on thelarge-scale channel properties can be more stably obtained from the CRS.On the contrary, since a DMRS is UE-specifically transmitted for aspecific scheduled RB and a precoding matrix, which is used by a basestation for transmission, may vary according to a PRG unit, an effectivechannel received by the UE may vary according to the PRG unit. Hence, ifa DMRS is used for estimating the large-scale channel properties of aradio channel over a wide band, performance degradation may occur. Incase of a CSI-RS, since the CSI-RS has a relatively long transmissionperiod and a relatively low density, if the CSI-RS is used forestimating the large-scale channel properties of the radio channel,performance degradation may occur.

FIG. 11 illustrates an active antenna system (AAS);

In a wireless communication system after LTE Rel-12, the introduction ofan antenna system utilizing AAS has been discussed. Since each antennain the AAS corresponds to an active antenna including an active circuit,an antenna pattern can be changed in order to adapt to a wirelesscommunication environment. Thus, in the AAS, interference can be reducedand efficient beamforming can also be performed.

Moreover, if the AAS is established in two dimensions (i.e., 2D-AAS), itis possible to adjust a beam direction at a main lobe of each antennanot only in the horizontal direction but also in the vertical directionin terms of the antenna pattern. Thus, the beam adaptation can beperformed more efficiently in three dimensions. In addition, it ispossible to actively change a transmitted beam depending on a locationof a UE based on the above beam adaptation. That is, the 2D-AAS may meanan antenna system having multiple antennas where the multiple antennasare installed in the vertical and horizontal directions.

When the above-mentioned 2D-ASS is introduced, a large number ofantennas may be installed in a vertical antenna domain and thus thenumber of antennas is remarkably increased. To efficiently manage such alarge number of the antennas, reference signal (RS) design for measuringa channel at each antenna and feedback design for a UE to providefeedback of channel information between each antenna and the UE becomesvery important. The reason for this is that as the number of antennasincreases, an RS overhead and a feedback overhead increases eitherlinearly or exponentially in general.

In the current LTE system, REs (resource elements) amounting to thenumber of antenna ports are assigned for a CSI-RS in each PRB (physicalresource block) pair. If 64 antennas are used as shown in FIG. 11 and areference signal is designed similar to that of the current LTE system,64 resource elements need to be assigned for the CSI-RS in each PRBpair. In addition, in the case of the normal CP (cyclic prefix),considering that 168 resource elements are present in the PRB pair, toomany resource elements are used for the CSI-RS. Moreover, in this case,resource elements that can be used for transmitting actual data aresignificantly insufficient in consideration of control channels andother reference signals.

To solve such an overhead problem due to the CSI-RS, methods forreducing a reference signal overhead by transmitting reference signalsonly through some antenna ports have been proposed. For instance,reference signal design that uses the Kronecker product has beenproposed. According to the reference signal design, in the case of anantenna arrangement with 8 rows and 8 columns shown in FIG. 11,reference signals are transmitted only through antennas included in asingle row and antennas included in a single column After receiving thereference signals, a UE performs the Kronecker product on channels forthe antennas in the row and column based on the received referencesignals. Thereafter, the UE may restore channels for the remainingantennas that are not used for transmitting the reference signals.Further, instead of the Kronecker product, the UE may use a differentmethod to estimate channel states of the remaining antennas based on thereceived reference signals with respect to some antennas.

When the channel states of the remaining antennas are estimated orrestored from the reference signals with respect to some antennas asdescribed above, the channel estimation may be performed inaccurately.In addition, the inaccurate channel estimation may cause a UE totransmit an inaccurate CQI (channel quality indicator) to a basestation. FIG. 12 illustrates a CQI-mismatch in the related art. Thegraph in FIG. 12 shows differences between channel qualities ofindividual channels when a PMI (precoding matrix indicator) obtainedfrom a channel restored using the Kronecker product is applied to theactual channel and the channel based on the Kronecker product.

FIG. 12 shows the CQI measurement performed by four UEs during 1000subframes. Referring to FIG. 12, the CQI mismatch is represented as aCQI error (dB) and the CQI mismatch between the channel restored by theKronecker product and the actual channel reaches a high level. Forinstance, the CQI mismatch measured in a first UE (UE1) exceeds 30 dB.Such a CQI mismatch may have an effect when the base station determinesscheduling and an MCS (modulation and coding scheme) level. Thus, theCQI mismatch may cause overall performance degradation in thecommunication system.

Thus, when all antenna channels are restored from the reference signalsfor some antennas as described above, a method for preventing the CQImismatch is required. Hereinafter, embodiments for preventing the CQImismatch will be described based on the above discussion.

Embodiment 1

A new reference signal can be used to reduce the aforementioned CQImismatch. For instance, the CQI mismatch may be corrected by designingthe new reference signal. First of all, a base station may receive a CSI(channel state information) report from a UE. In this case, the CSIreport transmitted from the UE may be state information on all channelsrestored from the reference signals for some antennas. After receivingthe CSI report, the base station may select a rank and a precoding to betransmitted to the UE in consideration of a PMI (precoding matrixindicator) and an RI (rank indicator) of the CSI. Thereafter, the basestation may generate ports as many as the selected rank by applying theselected precoding and then transmit new reference signals through thegenerated ports. The new reference signal according to the presentinvention may correspond to a reference signal used in the wirelesscommunication system supporting the 2D-AAS composed of multiple verticalantennas and multiple horizontal antennas and it can be defined toreduce the CQI mismatch. For example, the new reference signal accordingto the present invention can be defined to prevent CQI mismatches atselected ports. Here, ports may mean identification numbers of antennasor a logical or physical group of antennas. For instance, the selectedports may mean horizontal antenna ports included in one row or verticalantenna ports included in one column. In addition, the selected portsmay indicate at least one horizontal antenna port and at least onevertical antenna port included in a prescribed region.

When transmitting the new reference signals, the base station may informthe UE of the number of ports through which the new reference signalsare transmitted (e.g., a value equal to the rank currently selected bythe base station). In addition, the base station may inform the UE ofwhich CSI-RSs are associated to the new reference signals. For instance,the base station may inform the UE of the CSI-RSs associated with thenew reference signals by using a scheme of informing a QCL (quasico-located) CSI-RS identifier through PQI (PDSCH resource elementmapping and quasi co-location indicator) of the current LTE system. TheUE may calculate the CQI using the received new reference signals andthen feedback the calculated CQI value to the base station. In addition,instead of informing the associated CSI-RSs and the selected rank, thebase station may inform the UE of an MCS level and the rank using DCI(downlink control information). In this case, the base station may nottransmit separate data using the DCI.

The CQI value calculated from the above-described new reference signalsmay be transmitted through an existing CSI feedback procedure designedin the LTE system. Alternatively, the CQI value may be transmitted tothe base station through a new feedback procedure for CQI transmission.The UE may report the CQI value calculated from the new referencesignals to the base station. Moreover, the UE may report a differencebetween the CQI value calculated from the new reference signals and aCQI value calculated from the associated CSI-RSs. Furthermore, the newreference signal may be configured to have a longer period than theCSI-RS associated with the new reference signal.

Additionally, when the CQI feedback for correcting the CQI mismatchoverlaps with the conventional PMI/CQI feedback in the same subframe,the UE may provide only the CQI feedback for correcting the CQImismatch. Further, when the CQI feedback for correcting the CQI mismatchoverlaps with the conventional PMI/CQI feedback in the same subframe,the UE may encode the CQI feedback for correcting the CQI mismatchtogether with the conventional PMI/CQI feedback and then provide theencoded feedback. When the CQI mismatch is corrected using the newreference signals as described above, the base station may instruct, byusing DCI for uplink, the UE whether to report the CQI calculated fromthe new reference signals.

Embodiment 2

The CQI mismatch can be corrected using a DM-RS (demodulation referencesignal) currently used in the LTE system, instead of the new referencesignal. Description overlapping with the embodiment 1 will be omitted inthe following. For instance, after receiving the CSI report, the basestation may select the rank and the precoding to be transmitted to theUE based on the PMI (precoding matrix indicator) and the RI (rankindicator) of the CSI. Thereafter, the base station may generate portsas many as the selected rank by applying the selected precoding and thentransmit the DM-RS and a PDSCH through the generated ports. The UE mayobtain RI (rank indicator) information and an MCS (modulation and codingscheme) level for the corresponding UE based on DCI (downlink controlinformation). Thus, in this case, the base station does not need toinform the UE of the associated CSI-RS. After recalculating the CQIusing the DM-RS, the UE may feedback the recalculated CQI value to thebase station.

As mentioned in the foregoing description, the CQI value may betransmitted through the existing CSI feedback procedure used in the LTEsystem. However, a new feedback procedure may be used for CQI feedback.In this case, the UE may report the CQI value recalculated using theDM-RS to the base station. Moreover, the UE may report differencesbetween the CQI value and/or the MCS (modulation and coding scheme)level recalculated using the DM-RS and the CQI value and/or the MCSlevel included in the CSI report.

When the CQI mismatch is corrected using the DM-RS, the base station mayinstruct, by using DCI associated with the PDSCH transmitted togetherwith the DM-RS, the UE whether to report the CQI based on the DM-RS. Inaddition, if the base station instructs, by using DCI for downlink, theUE whether to report the CQI, the UE may not receive a resource and anMCS level for the CQI feedback. In this case, the UE may use the mostrecent uplink DCI to provide the CQI feedback.

Embodiment 3

The CQI value can be calculated using the CSI-RS. In other words, theCQI mismatch can be corrected using the CSI-RS used in the current LTEsystem. Description overlapping with the embodiments 1 and 2 will beomitted in the following. For instance, after receiving the CSI reportfrom the UE, the base station may select the rank and the precoding tobe transmitted to the UE based on the PMI (precoding matrix indicator)and the RI (rank indicator) of the received CSI. Thereafter, the basestation may generate ports as many as the selected rank by applying theselected precoding and then transmit the CSI-RS for correcting the CQImismatch to the UE through the generated ports. In this case, the basestation may inform the UE of which CSI-RS is associated with the CSI-RSfor correcting the CQI mismatch. In addition, when the CSI-RS forcorrecting the CQI mismatch is configured, the base station may informthe UE of information on the previous CQI.

After recalculating the CQI using the newly configured CSI-RS forcorrecting the CQI mismatch, the UE may feedback the recalculated CQIvalue to the base station. The CQI value may be transmitted through theexisting CSI feedback procedure used in the LTE system. However, therecalculated CQI value may be transmitted through a newly designedfeedback procedure. In this case, the UE may report, to the basestation, the CQI value calculated using the CSI-RS for correcting theCQI mismatch or a difference between the CQI value calculated using theCSI-RS for correcting the CQI mismatch and a CQI value calculated usingthe associated CSI-RS. Moreover, the CSI-RS for correcting the CQImismatch may be configured to have a longer period than the associatedCSI-RS. Furthermore, the base station may instruct, by using the DCI foruplink, the UE whether to report the CQI calculated using the CSI-RS forcorrecting the CQI mismatch to the base station.

The above-described embodiments may correspond to combinations ofelements and features of the present invention in prescribed forms. And,it may be able to consider that the respective elements or features maybe selective unless they are explicitly mentioned. Each of the elementsor features may be implemented in a form failing to be combined withother elements or features. Moreover, it may be able to implement anembodiment of the present invention by combining elements and/orfeatures together in part. A sequence of operations explained for eachembodiment of the present invention may be modified. Some configurationsor features of one embodiment may be included in another embodiment orcan be substituted for corresponding configurations or features ofanother embodiment. And, it is apparently understandable that a newembodiment may be configured by combining claims failing to haverelation of explicit citation in the appended claims together or may beincluded as new claims by amendment after filing an application.

In this disclosure, a specific operation explained as performed by aneNode B may be performed by an upper node of the eNode B in some cases.In particular, in a network constructed with a plurality of networknodes including an eNode B, it is apparent that various operationsperformed for communication with a user equipment can be performed by aneNode B or other network nodes except the eNode B. ‘Base station (BS)’may be substituted with such a terminology as a fixed station, a Node B,an eNode B (eNB), an access point (AP) and the like.

Embodiments of the present invention may be implemented using variousmeans. For instance, embodiments of the present invention may beimplemented using hardware, firmware, software and/or any combinationsthereof. In case of the implementation by hardware, one embodiment ofthe present invention may be implemented by at least one of ASICs(application specific integrated circuits), DSPs (digital signalprocessors), DSPDs (digital signal processing devices), PLDs(programmable logic devices), FPGAs (field programmable gate arrays),processor, controller, microcontroller, microprocessor and the like.

In case of the implementation by firmware or software, one embodiment ofthe present invention may be implemented by modules, procedures, and/orfunctions for performing the above-explained functions or operations.Software code may be stored in a memory unit and may be then drivable bya processor.

The memory unit is located at the interior or exterior of the processorand may transmit and receive data to and from the processor via variousknown means.

It will be apparent to those skilled in the art that the presentinvention can be embodied in other specific forms without departing fromthe spirit and essential characteristics of the invention. Thus, theabove embodiments are to be considered in all respects as illustrativeand not restrictive. The scope of the invention should be determined byreasonable interpretation of the appended claims and all change whichcomes within the equivalent scope of the invention are included in thescope of the invention.

INDUSTRIAL APPLICABILITY

The above-described embodiments of the present invention can be appliedto various mobile communication systems.

What is claimed is:
 1. A method of measuring a channel quality, which ismeasured by a base station equipped with a two-dimensional activeantenna system including a plurality of antennas, the method comprising:receiving, from a user equipment, CSI (channel state information) on theplurality of the antennas, wherein the CSI is generated based on a firstreference signal for a part of the plurality of the antennas; selectinga precoding and a rank based on a PMI (precoding matrix indicator) andan RI (rank indicator) of the received CSI; generating a port byapplying the selected precoding and the selected rank; transmitting, tothe user equipment, a DM-RS (demodulation-reference signal) and a PDSCH(physical downlink shared channel) configured for the user equipmentthrough the generated port; and receiving, from the user equipment, CQI(channel quality indicator) feedback for reducing a CQI mismatch of theCSI, wherein the CQI feedback is generated based on the DM-RS.
 2. Themethod of claim 1, wherein the CQI feedback comprises a CQI valuecalculated based on the DM-RS.
 3. The method of claim 1, wherein the CQIfeedback comprises differences between the CQI value and an MCS(modulation and coding scheme) level calculated based on the DM-RS and aCQI value and an MCS level calculated based on the first referencesignal.
 4. The method of claim 1, further comprising transmitting, tothe user equipment, information indicating presence of CQI feedbacktransmission by using DCI (downlink control information) associated withthe PDSCH.
 5. The method of claim 1, further comprising transmitting, tothe user equipment, information indicating presence of CQI feedbacktransmission by using DCI (downlink control information) for uplink. 6.A method of measuring a channel quality, which is measured by a basestation equipped with a two-dimensional active antenna system includinga plurality of antennas, the method comprising: receiving, from a userequipment, CSI (channel state information) on the plurality of theantennas, wherein in the CSI is generated based on a first referencesignal for a part of the plurality of the antennas; selecting aprecoding and a rank based on a PMI (precoding matrix indicator) and anRI (rank indicator) of the received CSI; generating a port by applyingthe selected precoding and the selected rank; transmitting, to the userequipment, a CSI-RS (CSI-reference signal) through the generated port;and receiving, from the user equipment, CQI (channel quality indicator)feedback for reducing a CQI mismatch of the CSI, wherein the CQIfeedback is generated based on the CSI-RS.
 7. The method of claim 6,further comprising transmitting the first reference signal associatedwith the CSI-RS to the user equipment.
 8. The method of claim 6, whereinthe CQI feedback comprises a CQI value calculated based on the CSI-RS.9. The method of claim 6, wherein the CQI feedback comprises differencesbetween the CQI value and an MCS (modulation and coding scheme) levelcalculated based on the CSI-RS and a CQI value and an MCS levelcalculated based on the first reference signal.
 10. The method of claim6, wherein a transmission period of the CSI-RS is longer than atransmission period of the first reference signal.
 11. The method ofclaim 6, further comprising transmitting, to the user equipment,information indicating presence of CQI feedback transmission by usingDCI (downlink control information) for uplink.
 12. A method of measuringa channel quality, which is measured by a user equipment in a wirelesscommunication system using a two-dimensional active antenna systemincluding a plurality of antennas, the method comprising: receiving,from a base station, a first reference signal for a part of theplurality of the antennas; generating CSI (channel state information) onthe plurality of the antennas based on the received first referencesignal; transmitting the generated CSI to the base station; receiving,from the base station, a DM-RS (demodulation-reference signal) and aPDSCH (physical downlink shared channel) configured for the userequipment through a port generated by applying a precoding and a rankselected based on the CSI; generating CQI (channel quality indicator)feedback for reducing a mismatch of the CSI based on the received DM-RS;and transmitting the generated CQI feedback to the base station.
 13. Themethod of claim 12, wherein the CQI feedback comprises a CQI valuecalculated based on the DM-RS.
 14. The method of claim 12, wherein theCQI feedback comprises differences between the CQI value and an MCS(modulation and coding scheme) level calculated based on the DM-RS and aCQI value and an MCS level calculated based on the first referencesignal.
 15. The method of claim 12, further comprising receiving, fromthe base station, information indicating presence of CQI feedbacktransmission through DCI (downlink control information) associated withthe PDSCH.
 16. The method of claim 15, wherein the CQI feedback istransmitted according to DCI (downlink control information) mostrecently received from the base station.
 17. A method of measuring achannel quality, which is measured by a user equipment in a wirelesscommunication system using a two-dimensional active antenna systemincluding a plurality of antennas, the method comprising: receiving,from a base station, a first reference signal for a part of theplurality of the antennas; generating CSI (channel state information) onthe plurality of the antennas based on the received first referencesignal; transmitting the generated CSI to the base station; receiving,from the base station, a CSI-RS (channel state information-referencesignal) through a port generated by applying a precoding and a rankselected based on the CSI; generating CQI (channel quality indicator)feedback for reducing a CQI mismatch of the CSI based on the receivedCSI-RS; and transmitting the generated CQI feedback to the base station.18. The method of claim 17, further comprising receiving, from the basestation, information of the first reference signal associated with thereceived CSI-RS.
 19. The method of claim 17, wherein the CQI feedbackcomprises a CQI value calculated based on the received CSI-RS.
 20. Themethod of claim 17, wherein the CQI feedback comprises differencesbetween the CQI value and an MCS (modulation and coding scheme) levelcalculated based on the CSI-RS and a CQI value and an MCS levelcalculated based on the first reference signal.